The IEEE 802.16m amendment allows a mobile station (MS) to transmit bandwidth requests (BWREQ) to indicate to a base station (BS) that it needs uplink (UL) bandwidth allocation. There are multiple methods by which a mobile station can request bandwidth from the base station. These methods include use of a contention based random access based bandwidth request indicator, a standalone bandwidth request, a piggybacked bandwidth request carried in an extended header in the MAC PDU, and a bandwidth request using fast feedback channel.
In the contention based random access method, multiple mobile stations contend for a limited set of preamble sequences on a common channel. The 802.16m standard defines 24 orthogonal access sequences, or preamble sequences. Each mobile station may choose a preamble sequence at random (out of the 24 possible preamble sequences), and hope that no other mobile station chooses that preamble sequence. If another mobile happens to choose the same preamble sequence, then the two preamble sequences are said to have collided at the BS.